With recent advance in information processing devices and internationalization, it is increasingly common to make a telephone call in a foreign language through a telephone application installed in a personal computer, for example. In view of this trend, methods have been disclosed for controlling speech signals from a non-native speaker of a certain language so as to make his/her speech easier to understand by a native speaker of the language. For instance, Japanese Patent No. 4942860 discloses a technology for generating a phoneme label corresponding to input speech through speech recognition using an acoustic model, converting the phoneme label according to a specific conversion table, and producing synthesized speech from the converted phoneme label.